Modern networks for mobile communication are organized in cells. For this reason, a cell search procedure will be initiated by a user terminal upon power on or in preparation for a potential handover. In Evolved Universal Terrestrial Radio Access (E-UTRA) networks, the cell search procedure is based on a synchronization process between a user terminal (or User Equipment, UE) and a base station (or eNodeB) of the E-UTRA network potentially serving the user terminal.
The synchronization process in the E-UTRA network involves Primary and Secondary Synchronization Signals (PSSs and SSSs) and provides frequency and symbol synchronization, frame synchronization and cell signature detection. The PSS and SSS jointly span a cell signature space. Each SSS additionally carries frame timing information indicative of whether the particular SSS is transmitted in sub-frame 0 or subframe 5. The frame timing information is exploited for frame synchronization. Still further, the SSS is indicative of a particular Cyclic Prefix (CP) configuration employed by the respective eNodeB.
According to Section 6.11 of 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 36.211 V.8.9.0 (2010-01), the cell signatures, also called Physical Cell Identities (PCIs), in an E-UTRA network are organized in 168 unique cell-identity groups (represented by an identifier NID(1)=0, . . . , 167), each cell-identity group comprising three cell-identities (represented by an identifier NID(2)=0, 1, 2), which amounts to a total of 504 PCIs (or cell IDs) NCELLID=3NID(1) NID(2) addressable by the pair of identifiers NID(1) and NID(2)).
Initially, the cell-identity identifier NID(2) is detected in a received PSS. Then, for detection of the associated cell-identity group identifier NID(1), an analysis of a received SSS XSSS is required. The analysis involves a correlation of the received SSS XSSS with SSS replicas indicative of all possible cell identity group identifier NID(1) hypotheses.
The correlation may be implemented according to
                                          c            ⁡                          (                                                N                  ID                                      (                    1                    )                                                  ,                                  s                  pos                                ,                                  cp                  type                                            )                                =                                    ∑                              n                =                0                            61                        ⁢                                                            X                  SSS                                      cp                    type                                                  ⁡                                  (                  n                  )                                            ·                                                d                                                            N                      ID                                              (                        1                        )                                                              ,                                          s                      pos                                                                            N                    ID                                          (                      2                      )                                                                      ⁡                                  (                  n                  )                                                                    ,                            (                  Eq          .                                          ⁢          1                )            to assess a hypothesized set of communication parameters including the cell-identity group identifier NID(1), two possible sub-frame positions spos (at 0 ms or 5 ms) and two possible cyclic prefix types cptype (normal or extended). Here, the received SSS assuming a cyclic prefix type cptype to be tested is denoted by XSSScptype. The SSS replica (or hypothesis signal) is denoted by
  d            N      ID              (        1        )              ,          s      pos            N    ID          (      2      )      for a given cell-identity group identifier NID(1) hypothesis, a given sub-frame position hypothesis spos and the cell-identity identifier NID(2) determined from the PSS (as SSS generation involves scrambling of the cell-identity group identifier NID(1) with the associated cell-identity identifier NID(2)).
The hypothesis signal
  d            N      ID              (        1        )              ,          s      pos            N    ID          (      2      )      is a sequence of elements in {−1, +1}, for which reason products in Eq. 1 between elements of the received SSS XSSScptype (n) and the hypothesis signal
  d            N      ID              (        1        )              ,          s      pos            N    ID          (      2      )      (n) are often not implemented by multiplication steps, but as fast sign changes. In the absence of multiplications steps, the computational complexity depends on the number of involved addition steps.
Computation of the correlations requires considerable hardware resources as the synchronization signals occur twice per 10 ms radio frame, and there are numerous combinations of communication parameters to be evaluated. The computational complexity of the implementation according to Eq. 1 requires 2×2×168×62=41.664 addition steps for evaluating each combination of the two cyclic prefix types (cptype=0, 1), the two sub-frame timings (spos=0, 1), and the 168 possible cell-identity groups (NID(1)), each evaluation involving 61 additions for correlating corresponding sequences of length I=62. After the correlations have been fully computed, the locations of correlation peaks that exceed a predefined threshold (and their magnitudes) are returned as cell-identity group candidates.
In E-UTRA networks, two different duplex modes are defined. Time Division Duplex (TDD) networks are operated synchronously, which means that the respective cells are aligned with respect to their frame timing. This also implies that synchronization signals from different cells may overlap at least partially in time. Frequency Division Duplex (FDD) networks operate either synchronously or asynchronously. It has been found that even in an asynchronous mode of operation, an at least partial overlap of synchronization signals from different cells may occur.
As a consequence of overlapping synchronization signals from different cells, the cell search procedure can be impaired. It has been observed in E-UTRA networks that cell search procedures become particularly error-prone in case of overlapping synchronization signals from “competing” cells utilizing the same PSS version (i.e., utilizing the same cell-identity identifier NID(2)).
One of the problems that impair the cell search procedure in the presence of overlapping synchronization signals, and in other cases, are correlation peaks that indicate the presence of “phantom” cells but are in reality only correlation artefacts. To reduce the detection probability of phantom cells a suitable threshold setting may be chosen for the proper identification of correlation peaks from a “valid” cell. However, since peaks for phantom cells may appear 3 to 4 dB below a peak of the corresponding valid cell, which is the same level as for potential neighbour cells to be detected, it is in practice difficult to find a suitable threshold setting: if the threshold setting is too high, valid cells (and in particular neighbor cells) may not be detected, while phantom cells will not be removed in case the threshold setting is too low.
In the presence of phantom cells, it may take more time to detect weaker cells potentially hidden behind a stronger cell. Under unfavourable conditions, the weaker cells may not be detectable at all. Additionally, processing power and battery resources will be wasted due to the extended cell search procedure.